This invention relates to the magnetic ballasting of granular, solid sorbents to increase their effective specific gravity and to render the sorbents magnetically recoverable from a liquid.
Another aspect of this invention relates to the use of magnetically ballasted solid sorbents in the fluidized bed extraction of selected constituents from a liquid.
A specific aspect of this invention relates to the sorption and recovery of heavy metal ions from aqueous solution using a multi-stage fluidized bed contactor.
Solid sorbents such as activated carbon granules and organic ion exchange resins are well known and have long been used in a large variety of extraction and purification processes. Specific examples of activated carbon use include the removal of organics from water as in the tertiary treatment of sewage effluents and the adsorption of gold from cyanide solutions. Ion exchange resins have found use in the selective extraction of metal ions from solution and, in the field of extractive metallury, are gaining prominence in the extraction and recovery of uranium from leach liquors.
Liquid-solid contacting systems used in sorption processes employing both activated carbon and ion exchange resins conventionally include fixed bed and pulsed bed, often referred to as moving bed, processes. In the fixed bed process, liquid is introduced into the top of a column loaded with sorbent and is dispersed evenly over the bed top by a distribution system.
Process liquid is removed from the bottom of the column. When the sorbent bed becomes exhausted, it is sluiced from the column for regeneration or disposal. The pulsed bed process operates similarly but periodically a portion of the sorbent is sluiced from the bottom of the column and an equal amount of fresh sorbent is introduced into the top of the column.
With whatever sorption media is used, these techniques suffer from uneven flow, or channeling, of liquid through the bed thus decreasing the efficiency of the process. In addition, these processes require that the liquid being treated be essentially free of suspended solids. The sorption media acts as a filter to remove suspended solids from the liquid resulting in plugging of the bed and/or severe channeling. These problems are compounded with certain ion exchange resins which swell significantly during the exchange process.
The process constraints imposed by fixed and moving bed sorption techniques have long been appreciated and efforts to develop fluidized bed sorption processes date back more than twenty years. Probably the most advanced of these is the NIMCIX system which is based on the work of Cloete and Streat as further developed by The National Institute for Metallurgy of Randberg, South Africa. Use of the NIMCIX system for the extraction and recovery of uranium using particular ion exchange resins has been successfully demonstrated on a relatively large scale. This work is described in the following two articles:
Haines, A. K., The Development of Continuous Fluidized-Bed Ion Exchange in South Africa, and Its Use in the Recovery of Uranium, Journal of the South African Institute of Mining and Mettalurgy, July 1978, pages 303-315. PA1 Craig et al, The Design and Operation of a Continuous Ion-Exchange Demonstration Plant for the Recovery of Uranium, Journal of the South African Institute of Mining and Metallurgy, July, 1978, pages 316-324.
Liquid flow velocities of about 30 cm/minute were achieved when operating on a liquid having a suspended solids content of 200 to 500 ppm. In addition, some preliminary work was carried out on slimes in which it was determined that the solids content of the liquid was limited to about 10% by the density of the commercially available resins. Higher solids content causes entrainment of the resin granules in the liquid. The Haines article also notes that attempts have been made to produce ion exchange resins of higher specific gravity but that "the heterogeneous materials that were synthesized have not achieved commercial success owing to problems of chemical and physical instability."
Activated carbon having magnetic properties is also known as is described in U.S. Pat. No. 3,803,033. That patent discloses powdered activated carbon having particles of magnetic iron oxide (Fe.sub.3 O.sub.4) adhering to the carbon. It is used for the adsorption of organic contaminants from water streams, such as sewage effluent, by slurrying the carbon in the water and thereafter removing carbon from the water by magnetic means.
Dawson et al in U.S. Pat. No. 4,134,831 discloses adsorbents and ion exchange media having attached thereto or incorporated therein magnetically susceptible materials such as ground natural magnetite. Patentees also suggest use of iron filings as the magnetic material but it must be noted that iron filings would very quickly react in most process environments to form either very weakly magnetic or non-magnetic ferrous and ferric oxides. The magnetic media is proposed for use in such processes as the underwater mining of minerals by dispersing the media in the bottom sediments of the water body and thereafter collecting the media by drawing a magnet through the water.